Best battery for the money to pair with a ccell cartridge?

I'd like to start making cartridges in bulk. I know the c cell is the best cartridge on the market right now and the cost difference is minimal compared to the old ce3 cartridges. What I'm looking for is a cheap battery that will run it well I can get in bulk and in a perfect world, it would use micro USB and not some proprietary charger.

any recommendations and sources for the recommended batteries?